1. Field of the Invention
The present invention relates to a mold assembly for producing a thin molded glass article with a large diameter and which does not require after-processing such as polishing, and more particularly to a mold assembly adapted for producing an optical disc substrate with micropatterns such as microgrooves and micropits, and a lens having a large aperture such as a spectacle lens.
2. Description of the Related Art
JP-A-203732/1984 discloses a prior-art process for molding a glass article having a high accuracy and which process does not require after-processing such as polishing after a press working. In accordance with this molding process, a glass to be molded is heated and softened on a tray and then the resulting softened glass gripped with a gripper is transferred into a mold assembly which presses the softened glass. However, in accordance with this molding process, gripping the softened glass by the gripper, and separating the glass which has been softened and adhered to the tray both cause deformation of the softened glass, so that a subsequent pressing cannot provide a satisfactory lens.
JP-A-118641/1985 discloses another prior-art process comprising the steps of supporting a glass material to be molded on a ring-shaped mold, heating a combination of the ring-shaped mold and the glass material to be molded and softening the latter, moving said combination into an assembly of an upper mold or cope and a lower mold or drag, and pressing the glass material to be molded. This latter prior-art process has solved the problems in the first mentioned prior-art process. However, in the latter prior-art process, once the glass preform to be molded which has been supported on the ring-shaped mold is softened, the softened glass preform deforms by its weight, and as the result, the glass preform sags and falls from the ring-shaped mold. Since a central area of the glass preform is recessed during a heating operation, the top surface of the glass preform has a tendency to produce a gas trap. Thus, the range of the molding condition is narrow and the molding operation has been sometimes unstable. In particular, when a diameter of a molded glass product is large or a thickness of the molded glass product is small, a relatively thin glass preform must be used, so that the latter prior-art process prevents sagging of the heated glass preform only with great difficulty.